how would I change the camera viewpoint or scene object and output the image at the runtime
Well there are 2 options you have here.
1. Optimize all your materials and bake all static shadows. That way the rendering will take as little time as possible. 
2. Once you load the scene, move the camera to all locations you want to render but don't render, just add keyframes. When you add keyframes to all locations then start to render the number of frames you need. That way you will only prepare for rendering and load into memory once and then render everything you need in one pass.